Galaxies in the visible universe

[Axel Boldt]

Is the number of galaxies in the visible universe increasing or
decreasing over time? The visible universe itself is clearly
increasing in size, but at the same time, the galaxies at its border
are moving away from us, so which effect is more important?

--
Axel Boldt ** ax...@uni-paderborn.de ** math-www.uni-paderborn.de/%7Eaxel/

[Chris Hillman]

On 14 Jan 1998, Axel Boldt wrote:

> Is the number of galaxies in the visible universe increasing or
> decreasing over time?

Increasing, but this is a subtle effect, the "newly visible" galaxies are
very distant and very old.

This is easiest to see if you use cylindrical conformal coordinates for
the Friedmann solution as sketched in my "Coordinate Tutorial", which is
archived on my web page

http://www.math.washington.edu/~hillman/relavitity.html

You can see this using the football model for a negative energy Friedmann
solution: take at two events on a single radius line in orthoprojection
coordinates (corresponds to a longitude line on the football) and look at
the "absolute past" (analog of past light cone for curved space). It is
bounded by cardioid curves, and the angle at which the bounding curves
open up is greater for the absolute past of the later event.

Hope this helps,

Chris Hillman

[Axel Boldt]

Chris Hillman <hil...@math.washington.edu> writes:

Thanks, I'll check that out and try to understand it. One more thing:
will we eventually see all galaxies (assuming an open, expanding FRW
model), or is there such a thing as a "never observable" corner of the
universe?

Thanks alot,